In order to extrude a constant amount of rubber or plastic material, it has been a conventional practice to use a combined system wherein a gear pump is arranged on a downstream side of an extruder, and to drive the gear pump independently from the extruder. However, such a combined system requires considerable space for installation. Thus, there has been proposed an extruder system provided with a gear pump, wherein the drive shaft for the extrusion screw is simultaneously used to drive the gear pump. Attention is drawn, e.g., to the pamphlet of International Publication WO 2003/011561.
FIG. 1 is a fragmentary longitudinal-sectional view showing the known extruder system in a plane including the rotational axis of the extrusion screw, and FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1. The extruder system 80 includes an extruder 81 for axially extruding a rubber or plastic material G in a barrel 83 by rotation of an extrusion screw 82 arranged in the barrel, and a gear pump 91 driven by a rotational force for rotating the extrusion screw 82 so that a predetermined amount of the material G extruded from the extruder is discharged by engagement of gears. The gear pump 91 includes a casing 93 that is secured to the barrel 83, a ring gear 94 fixedly secured to the radially inner side of the casing 93, and a rotatable wall body 85 secured to the extrusion screw 82.
The rotatable wall body 85 includes two side plates 85a, 85b extending perpendicularly to the rotational axis of the extrusion screw 82, and partition walls 85c for dividing the space between the side plates 85a, 85b in the circumferential direction, for example, into four chambers 87 as shown. Each chamber 87 is provided therein with a planetary pinion 86 that is rotatable about an axis in parallel with the rotational axis of the extrusion screw 82. Each planetary pinion 86 is arranged so as to divide the chamber 87 into a suction sub-chamber and a discharge sub-chamber, and meshed with the ring gear 94. Thus, when the extrusion screw 82 is rotated, the rotatable wall body 85 is rotated to cause an orbital revolution of the planetary pinions 86. On this occasion, being meshed with the ring gear 94, the planetary pinions 86 undergo a planetary rotation in a direction opposite to the direction of the orbital revolution.
Among the two side plates 85a, 85b, the side plate 85a is situated on the suction side adjacent to the extrusion screw 82, and the side plate 85b is situated on the discharge side remote from the extrusion screw 82. The side plate 85a on the suction side is formed with suction ports 88a which are in communication with the respective suction sub-chambers 87a, while the side plate 85b on the discharge side is formed with discharge ports 88b which are in communication with the respective discharge sub-chambers 87b. 
In operation of the known extruder system 80 described above, the material G extruded by the rotation of the extrusion screw 82 is passed through the suction port 88a in the side plate 85a on the suction side, fed into the radially outer side of the suction sub-chamber 87a, and forced into the gap between the neighboring teeth of the planetary pinions 86. As the planetary pinion 86 undergoes planetary rotation, while being retained in the gap between the neighboring teeth of the planetary pinion 86, the material G is successively moved to the radially inner side of the suction sub-chamber 87a, the radially inner side of the discharge sub-chamber 87a, and the radially outer side of the discharge sub-chamber 87a. When the material G reaches the radially outermost position in the discharge sub-chamber 87b, where the planetary pinion 86 comes into engagement with the ring gear 94, the material G is discharged through the discharge port 88b toward the axial tip end of the extruder system, due to the engagement between the planetary pinion 86 and the planetary pinion 86. On the other hand, the gap of the planetary pinion 86, from which the material has been discharged, is moved to the suction sub-chamber 87a so as to be filled with a new batch of the extruded material G
In this way, the volume of the material G that can be discharged by advancement of the planetary pinion 86 by one pitch is determined by the teeth profile of the planetary pinion 86, and it is possible to maintain a constant discharge volume of the material G by maintaining the rotational speed of the extrusion screw 82 constant.